Well that's not good. I assume you are using 5.0.5, if not, you should be. 5.0.5 has been out for almost a month being used for hundreds of flights per day and we haven't heard of this issue.
I just did a couple of tests with a Mavic 2 Pro and my iOS device in Airplane mode and it works as expected.
This is not to say there could not be an issue. Terrain Awareness should still totally work while you are offline as long as the data is already present.
I am not sure what you mean by "the iPad is connected to EITHER the drone or to WiFi/Internet". The app can totally be connected to the internet while connected to the aircraft. I guess the only exception here is for with the P3 Standard (which sounds like might be the case). While we still technically support the P3 Standard we haven't tested with it in years. It sounds like all of this connection juggling is causes trouble. This may be the issue. That would also explain why we haven't had any reports of it since there aren't too many of those out there doing work any more.
There are some tests in the software to see if there is a network connection. Since you are connected to the aircraft via WiFi that answer is technically a "yes" so it might be trying to get some data from the internet since the connection is there (not necessarily terrain data since that is already present). Even though the connection isn't a real one. This could be a Phantom 3 Standard specific issue since Map Pilot Pro had a lot of rework done to the Terrain Awareness system.
Friday night is not a great time to debug issues and get out bug fixes though... If this is in fact a Phantom 3 Standard specific issue we would have to find one somewhere to test with.
At this time the only workaround would be to not use the Terrain Awareness system when flying. You can use it to figure out what height you would need to fly to do so safely. Between the Terrain Awareness plot and the Ground Offset you could set the data collection to be 70% and some amount above the highest point in the flight so everything that is lower than that highest point would get more overlap than your setting and your map would turn out fine. Or you could get a newer aircraft (just a suggestion) and be up and running immediately.
We will get this fixed or figured out for you ASAP and adjust your subscription expiration date to end one year from when the issue has been addressed. Best case would be having something out in the middle of next week.